We have extended the analysis of X-ray crystallographic technique of the three-dimensional structure of an L-arabinose-binding protein from E. coli to 2.8 A. L-arabinose-binding protein is a member of a family of receptor or binding proteins; these proteins are essential components of active transport and chemotaxis in bacteria. The L-arabinose-binding protein structure is the first to be determined and thus provides a basis for analysis of the biological activities of the protein and for comparison with other receptor proteins. The L-arabinose-binding protein molecule consists of two similar domains with extensive secondary structures. The structure may have been solved with bound L-arabinose. The sugar-binding site is located in the cleft formed between the two domains. In order to unambiguously locate the sugar-binding site, the electron-rich substrate-analog 6-bromo-6-deoxy-D-galactose was synthesized and used in a difference Fourier analysis.